feat(cli): add json test reporter

[caspervonb]

This adds JSON as an output format for test results in the same style as the linter (non streaming).

Closes #8550

[nayeemrmn]

Discussed on discord. The output of --json shouldn't be events-based, it should print out a summary at the end. We should have a separate --ndjson/--jsonl/etc. flag which outputs test events as individual line-delimited JSON objects. If anyone thinks we only need one of these, we should favour the latter. I think both are useful.

This pattern of having an overarching JSON object where nested array elements are chunked one-by-one isn't a real format and doesn't work well. Streaming consumers can't take advantage of the overarching JSON structure and will have a really annoying time parsing. Non-streaming consumers have no use for an events-based output and likely just need a summary. So this output doesn't serve anyone.

[caspervonb]

Punting this for a while because upcoming changes to the internal representation is going to affect the output.

[caspervonb]

Closing this for now, will revisit later.